The present invention relates generally to a stainless steel alloy bipolar plate for use in a fuel cell environment that exhibits ease of manufacturability, and more particularly to such a bipolar plate that is easy and inexpensive to manufacture while preserving the best mechanical/structural properties possible.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) is typically disposed to form on these respective sides an anode to facilitate hydrogen oxidation and as a cathode to facilitate oxygen reduction. From this, electric current is produced with high temperature water vapor as a reaction byproduct. In one form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell, an electrolyte in the form of an ionomer membrane is situated between the anode and cathode to form a membrane electrode assembly (MEA) which is further layered between diffusion layers that allow both gaseous reactant flow to and electric current flow from the MEA. The aforementioned catalyst layer may be disposed on or as part of the diffusion layer or the membrane.
To increase electrical output, individual fuel cell units are stacked with bipolar plates disposed between the diffusion layer and anode electrode of one MEA and the diffusion layer and cathode electrode of an adjacent MEA. Typically, the bipolar plates are made from an electrically-conductive material in order to form an electrical pathway between the MEA and an external electric circuit. In such a stacked configuration, the bipolar plates separating adjacently-stacked MEAs have opposing surfaces each of which include flow channels separated from one another by raised lands. The channels act as conduit to convey hydrogen and oxygen reactant streams to the respective anode and cathode of the MEA, while the lands, by virtue of their contact with the electrically conductive diffusion layer that is in turn in electrical communication with current produced at the catalyst sites, act as a transmission path for the electricity generated in the MEA. In this way, current is passed through the bipolar plate and the electrically-conductive diffusion layer.
Because the bipolar plate operates in a high temperature and corrosive environment, conventional metals, such as plain carbon steel, may not be suitable for certain applications (such as in automotive environments) where long life (for example, about 10 years with 6000 hours of life) is required. During typical PEM fuel cell stack operation, the proton exchange membranes are at a temperature in the range of between about 75° C. and about 175° C., and at a pressure in the range of between about 100 kPa and 200 kPa absolute. Under such conditions, plates made from alloyed metals such as stainless steel may be advantageous, as they have desirable corrosion-resistant properties. In situations where cost of fuel cell manufacture is an important consideration, metal-based bipolar plates may be preferable to other high-temperature, electrically conductive materials, such as graphite. In addition to being relatively inexpensive, stainless steel plates can be formed into relatively thin members (for example, between 0.1 and 1.0 millimeters in thickness).
Of the various types of stainless steels, those with ferritic microstructures, which typically have a high chromium content and are typically devoid of nickel, exhibit body-centered cubic (BCC) crystal structure and tend to have the desirable attributes of being relatively low cost and high in corrosion resistance (the latter due to chromium oxide barrier formation). Nevertheless, their hardening curves are such that they are more susceptible to necking, stretching, thinning and consequent cracking when exposed to conventional stamping or related one-step metal-forming operations than their more conventional (for example, 304 stainless steel) counterparts. These difficulties are especially prevalent in single-step deep-draw operations (for example, those involving relative large—such as between about 300 microns and 400 microns in depth—out-of-plane deformations) where significant side wall deformation may take place. This early necking and fracture is especially prevalent in tight radii used to form the adjacent walls of the reactant flow channels. While the hardening curves of other more formable stainless steels (such as the aforementioned austenitics) generally allows for the more harsh bending conditions imposed by the conventional one-step approach, early necking and fracture from such single-step forming is also prevalent in situations where the draw depth is comparatively large (such as between about 400 and 500 microns, or more).
Moreover, current bipolar plate manufacturing accounts for high portion of overall fuel cell stack cost. While using stamped stainless steel bipolar plates would be beneficial in addressing a significant portion of this cost, the low formability of stainless steel in general (and ferritic stainless steel in particular) is a significant challenge, especially for stamping very thin (for example, 0.100 millimeters or thinner) sheets that are possessive of the required channel strength and depth to satisfy functional requirements.
To improve the formability of thin stainless steel sheet, a hydro-forming process could be used. Nevertheless, such a process is slow, and requires expensive special equipment that would make it hard to meet either the required production rate or production cost. Likewise, electro-magnetic forming could be used, but is a process that is still under development and not suitable for low-cost mass production.